1. Field of the Invention
The present invention relates to a switched power supply, and more particularly relates to a switched power supply using ringing choke converter (RCC) system.
2. Description of the Related Art
Household appliances, such as video cassette recorders (VCR) or fax machines, require a DC power supply that provides stable voltage drawing excessive amounts of power. Due to efficiency and relatively simple structure, switched power supplies using RCC system are widely utilized as for household appliances, transforming household AC power to the DC power required by the appliance circuits.
An RCC system, or RCC structure, has been disclosed in many patents such as in U.S. Pat. No. 6,081,433, U.S. Pat. No. 6,101,103, and U.S. Pat. No. 6,072,702.
FIG. 1 is a simple perspective view of a conventional RCC. AC represents external AC power. Through a diode bridge DB and a filtering capacitor C1, AC is transformed into high voltage DC power at node 1 to act as the main power for the RCC structure.
RCC structure comprises a transformer T, a switch component FET Q1, a starting resistor RS, a positive feedback circuit 40, a control circuit 46, and an output rectifier 42. The transformer T has at least three windings; a primary winding N1, a secondary winding N2 with an opposite polarity to N1, and a feedback winding Nb with the same polarity as N1. The switch component FET Q1 is connected in series to terminal 2 of the primary winding N1. The starting resistor RS is connected between terminal 1 of the primary winding N1 and the gate of the switch component FET Q1. The positive feedback circuit 40 comprises a resistor R4 and a capacitor C3, both connected in series between terminal 3 of the feedback winding Nb and the gate of the switch component FET Q1. The control circuit 46 is connected between terminal 3 of the feedback winding Nb and the gate of the switch component FET Q1. The output rectifier 42 has a serial connected diode CR51 and a parallel connected capacitor C51. The anode of the diode CR51 is connected to terminal 5 of the secondary winding N2.
When high voltage at terminal 1 of the primary winding N1 occurs, the resulting current through the RS gradually charges the gate of the FET Q1. When the voltage difference between the gate of the FET Q1 and the source of the FET Q1 reaches a threshold voltage Vt of the FET Q1, the FET Q1 is activated to conduct current through terminal 1 and terminal 2 of the primary winding N1. The current change between terminal 1 and terminal 2 generates an induction voltage between terminal 4 and terminal 3 of the feedback winding Nb. Via a coupling effect of positive feedback circuit 40, the induction voltage increases the voltage of the gate of the FET Q1 and, as a result, increases the current value of the current between terminal 1 and terminal 2. This positive feedback continues to increase the current value of the current between terminal 1 and terminal 2, and stores sufficient current energy at primary winding N1.
The resistor R5 and the capacitor C5 of the control circuit 46 consist of an RC delay circuit. When the capacitor C5 is charged to a certain level, the transistor Q3 is activated to decrease the voltage of the gate of the FET Q1 and thereby deactivate the FET Q1. At switching, the current energy stored at the primary winding N1 is transferred to the secondary winding N2 and the feedback winding Nb. An induction current at the secondary winding N2 charges the capacitor C51, and provides power to the external circuit via terminal Vo. Feedback winding Nb, resistor R4 and capacitor C3 construct an LC oscillator. When the voltage at terminal 3 oscillates and is converted from a negative value to a predetermined positive value, via the coupling of the capacitor C3, the FET Q1 is activated again and stores the current energy at the primary winding N1. Through repeated cycles, the primary winding N1 continues to transfer the current energy to the secondary winding N2.
Though the above-described RCC structure oscillates, it does not guarantee a fixed voltage difference between the Vo and the GND. In other words, if the secondary winding N2 continues to charge C51, it is possible that the resulting high voltage difference between the Vo and the GND may damage the circuit connected between the Vo and the GND.
Accordingly, most of the RCC structure further includes a detect circuit 48 comprising a light emitting diode PD and a zener diode ZD, connected between two terminals of the capacitor C51, as shown in FIG. 1. When the voltage difference of the capacitor C51 is higher than the breakdown voltage of the zener diode ZD, the light emitting diode Pd emits light. It follows that the photo-transistor PT of the control circuit 46 is activated by the received light from the light emitting diode PD. The process is served to decrease the time required for charging the capacitor C5 to activate the transistor Q3. The process avoids overloading of current stored at the primary winding and maintains an acceptable voltage level between the Vo and the GND.
However, if the AC voltage experiences a shortage, that is, if the main power voltage of the RCC structure decreases, the induction voltage generated at terminal 3 of the feedback winding Nb also decreases. Consequently, the control circuit 46 does not have sufficient voltage to activate the Q3 and is not able to deactivate FET Q1. Thus, FET Q1 may be continuously activated, wasting electrical power.
As a result, the main object of the present invention is to provide a switched power supply using RCC system that prevents the problem of power waste when AC voltage is in limited supply.
Another object of the present invention is to avoid unnecessary power output when the output load is excessive.
Another object of the present invention is to efficiently avoid excessive output voltage.
Still another object of the present invention is to efficiently decrease the power consumption of the switched power supply when there is no output load to meet environmental concerns.
Based on the above mentioned objects, the present invention provides a switched power supply comprising a transformer, a switch component, a rectifier circuit, a positive feedback circuit, a control circuit and a regulated control circuit. The transformer includes at least a primary winding, a secondary winding and a feedback winding. The switch component includes a main control terminal and is connected in series to the primary winding. The rectifier circuit is connected to the secondary winding and used to output DC voltage. The positive feedback circuit is connected between the first terminal of the feedback winding and the main control terminal. When the switch component is switched to the On position, the positive feedback circuit provides the main control terminal with a positive feedback voltage. The control circuit includes a first control component and a delay circuit, and is connected between the feedback winding and the main control terminal. The first control component is connected between the control terminal and a first ground and has a first control terminal. The delay circuit is connected between the first terminal and the first control terminal. After the switch component is switched to the On position for a predetermined interval, the feedback winding receives a first voltage to switch the first control to the On position and thereby switch the switch component to the Off position. The regulated control circuit is connected between a second terminal of the feedback winding and the first control terminal. When the DC voltage reaches a first predetermined voltage value, the regulated control circuit provides a second voltage to switch the first control component to the On position and thereby switch the switch component to the Off position.
The present invention further provides a method for controlling a switched power supply. The switched power supply comprises a transformer, a switch component and a rectifier circuit. The method comprises the following steps: (1) providing a main control terminal of the switch component with a positive feedback voltage by the positive feedback circuit when the switch component is switched to the On position; (2) providing the first control component with a first voltage by the first terminal to switch the switch component to a off state after the switch component is switched to the On position for a predetermined interval; and (3) providing the first control component with a second voltage by a second terminal of the feedback winding to switch the switch component to a off state, when the DC voltage reaches a first predetermined voltage value.
The advantage of the present invention is that it provides an efficient apparatus and method for under-voltage protection (UVP), over-voltage protection, and overload protection. In addition, the present invention provides features to efficiently decrease power consumption of the RCC structure when the RCC structure is not loaded.